Re: [Jack-Devel] How does --hwmix work?

"Chris Caudle" &lt;chris(a)chriscaudle.org&gt; writes: [Large treatise regarding hardware monitoring elided] Uh, the question was not what hardware monitoring is. The question was what the --hwmon option to Jackd does and which applications support it. "the hardware is responsible for mixing the incoming audio material" is nothing that Jackd would be involved with since Jackd isn't hardware. So what does this option do? The manual page reads (as you cited): -H, --hwmon Enable hardware monitoring of capture ports. This is a method for obtaining "zero latency" monitoring of audio input. It requires support in hardware and from the underlying ALSA device driver. When enabled, requests to monitor capture ports will be satisfied by creating a direct signal path between audio interface input and output connectors, with no processing by the host computer at all. This offers the lowest possible latency for the monitored signal. Presently (March 2003), only the RME Hammerfall series and cards based on the ICE1712 chipset (M-Audio Delta series, Terratec, and others) support --hwmon. In the future, some consumer cards may also be supported by modifying their mixer settings. Without --hwmon, port monitoring requires JACK to read audio into system memory, then copy it back out to the hardware again, imposing the basic JACK system latency determined by the --period and --nperiods parameters. But what does this mean when I have 26 capture ports and 18 output ports? What kind of applications request to "monitor capture ports", and what ports will get a direct signal path between them? When I use Ardour, I have settings for "Ardour is responsible for monitoring" and "Hardware is responsible for monitoring". For which of the two settings would the --hwmon option to Jackd be even relevant? Wait. Now I see _three_ settings. Record monitoring handled by: "via Audio Driver" "ardour" "audio hardware" [Testing out several things] Ok, the "via Audio Driver" option is not there when Jackd runs on my default laptop hardware (AC97) but it is there when it runs on the RME Hammerfall DSP. But independently of whether or not I specify --hwmon. So does this option actually make a difference to Jackd? Either way, what is the effect? Does Ardour take the gain totals (disregarding all other plugin effects) from its jackd-controlled hardware inputs to its record-enabled tracks and from its record-enabled tracks to its jackd-controlled hardware outputs, convert them to mixer settings and hand them over? That would be what I consider "best case". It is also something about which I can find nothing whatsoever in the Jack API documentation (as seen on its web site). So it's hard to guess at the actual capabilities being exercised here. If it accesses the mixer like that, it's actually a bit more than just monitoring. Who does the latency accounting in this case? The Hammerfall docs state 4 samples delay at 48kHz and its ilk and 8 samples delay at 96kHz and its ilk for the digital signal paths. For the analog signals, A/D has 31 samples delay due to oversampling/filtering and D/A has 30 samples delay. So basically Ardour's menu "Editor/Audio/Monitoring/Record monitoring handled by:" gains an additional option "via Audio Driver" on certain hardware, independently of whether jackd has been started with --hwmon or not. This magic third option is not mentioned in the Ardour manual <http://manual.ardour.org/preferences-and-session-properties/preferences-dialog/audio/>: Record monitoring handled by: determines whether Ardour provides monitoring of incoming audio or whether monitoring is provided by hardware. See Monitoring(http://manual.ardour.org/recording/monitoring/) for more information. And "Monitoring" is actually less rather than more information: When recording, it is important that performers hear themselves, and to hear any pre-recorded tracks they are performing with. Audio recorders typically let you monitor (i.e. listen to) the input signal of all tracks that are armed for recording, and playing back the unarmed tracks. So does --hwmon make an actual difference (I should likely mention that I used Jack2 for this experiment)? And what actually happens? And is there any documentation about any of this and the involved APIs anywhere? And if this requires special Jackd support (as the manual page suggests) rather than being able to work with more generic ALSA-accessible mixers (like, say, the AC97 mixer), where would one find out about just which cards are supported in 2017? The manual page states "Presently (March 2003)" which is not particularly up to date. -- David Kastrup